In the OFDM/TDMA-system as shown in FIGS. 1-4, a plurality of subcarriers 1, which are orthogonal to each other, can be allocated to a variable number of channels U0, U1 . . . U9, each channel U0, U1 . . . U9 containing a variable number of subcarriers 1 depending on the information to be transmitted. FIG. 1 shows a group of ten frequency channels U0, U1 . . . U9. Each frequency channel U0, U1 . . . U9 can contain a variable number of subcarriers depending on the information to be transmitted, as shown for the channels U0 and U1 in FIG. 2. The channel U0 contains a plurality of subcarriers 1, and the channel U1 contains a number of subcarriers different from channel U0. The number of subcarriers 1 allocated in each channel depends on the amount of information to be transmitted. The channel U0 shown in FIG. 2 contains 21 subcarriers, whereas the channel U1 shown in FIG. 2 contains only 10 subcarriers. Therefore, the channel U0 can be transmitted at more than twice the transmission rate of the channel U1. On the border of each channel U0, U1 . . . U9, a single subcarrier having zero power is placed as a guard band 2 to minimize interference to subcarriers placed in the adjacent frequency band or to fulfill certain spectrum masks. If interference from the neighboring band is small, the guard band 2 need not to be provided, whereas, if the interference is excessive, a plurality of guard bands 2 can be provided.
The subcarriers 1 are generated by orthogonal frequency division multiplex (OFDM) processing. As shown in FIG. 3, W(f) indicates a wave form indicating an energy on the frequency axis and B(Hz) indicates the distance between two adjacent subcarriers. The OFDM processing provides for a multi-subcarrier-system, wherein the number of channels which can be multiplexed is not limited by interference from the other channels and can be freely determined depending on the bandwidth to be allocated. By changing the number of subcarriers to be allocated to the different channels, it is possible to change the transmission rate or to achieve a variable transmission rate. The subcarriers between the respective channels can be easily separated by means of a filter, thereby making it possible to prevent deterioration of S/N characteristics. Since OFDM processing is used for multi-subcarrier modulation, a guard band S is not necessarily needed between different channels, thereby achieving very high spectral efficiency. Further, because fast Fourier transformation can be utilized, the necessary processing can be rapid and small.
The number of channels in each group of channels can be varied, as shown in FIG. 4, in which a group of six channels U0, U1 . . . U5 is shown. In the OFDM/TDMA-system, the number of channels in a group can be varied within the system frequency band depending on information to be transferred.
In the known and standardized GSM-System, a type of single carrier frequency modulation called GMSK is used. The frequency channels are constant and the spacing between adjacent frequency channels is 200 kHz. The number of FDMA-channels is 124 and time division multiple access (TDMA) is used to support the number of parallel connections. The TDMA scheme in the GSM-System is 8 GSM-timeslots within one time frame. The GSM-timeslot length is 576.9 xcexcs (15/26 ms), as is shown in FIG. 5. As can be seen in FIG. 5, the transmitted GSM-timeslots are not fully occupied by the transmitted burst to reduce interference from adjacent GSM-timeslots if the system is not perfectly synchronized. The guard period is 8.25 bits, which corresponds to 30.5 xcexcs. The guard period is divided in two parts, wherein one of the parts is located at the beginning of the GSM time slot, and the other part is located at the end of the GSM-timeslot.
A GSM time frame consists of 8 GSM time slots and has therefore a length of 4615.4 xcexcs, as is shown in FIG. 6. The GSM-system supports slow frequency hopping, which is explained in FIG. 6. The shown GSM-timeslot 3 is a receiving timeslot. According to the time division duplex (TDD)-system of the GSM-system, a corresponding transmission GSM-timeslot 4 is transmitted some timeslots later. Further on, the GSM-system makes use of the frequency division duplex (FDD)-system with 45 MHz between uplink and downlink, so that the transmission GSM-timeslot 4 is transmitted in the corresponding uplink frequency band, when the receiving GSM-timeslot 3 had been sent in the downlink frequency band, or vice versa. The next succeeding receiving GSM-timeslot 5 is of course transmitted in the same uplink or downlink frequency band as the preceding GSM-timeslot 3, but in a different frequency channel, according to the slow frequency hopping. The frequency hopping improves, together with the interleaving procedure, the transmission of the signals in view of the frequency and interference diversity. The usual interleaving depth in the GSM-system is 36.923 ms corresponding to 8xc3x978 GSM-timeslots.
An object of the present invention is to provide a technique for transmitting signals on the basis of the OFDM/TDMA-system, where in the signals can be transmitted in a GSM-system.
Signals transmitted on the basis of a OFDM/TDMA-system by allocating a plurality of subcarriers, which are orthogonal to each other, to a variable number of channels, each channel containing a variable number of subcarriers depending on the information to be transmitted.
For transmission in a GSM-system having a constant number of predetermined GSM-frequency channels and a constant number of predetermined GSM-timeslots grouped in GSM-frames, the number of subcarriers allocated to each of the GSM-frequency channels is such that a multiple of one resulting OFDM/TDMA-timeslot matches with one or a multiple of one GSM-timeslot.
The number of subcarriers to be allocated to one GSM-frequency channel preferably is chosen so that several OFDM/TDMA-timeslots are mapped into one GSM-timeslot, or several OFDM/TDMA-timeslots are mapped into several GSM-timeslots, e.g. into eight GSM-timeslots (one GSM-frame). In the OFDM/TDMA-system, the transformation of one or a plurality of the subcarriers into the time domain results in an OFDM/TDMA-time burst. According to the present invention, one OFDM/TDMA timeslot contains essentially one OFDM/TDMA-time burst.
An important consequence of the mapping of the OFDM/TDMA-timeslots into the GSM-timeslots that the same interleaving depth as in a standard GSM-system can be obtained. A standard GSM-interleaving depth is 8xc3x978 GSM-timeslots (approximately 36.923 ms). In the present invention, one or more OFDM/TDMA-timeslots (e.g. two, four, . . . ) are mapped into one GSM-timeslot. Therefore, the information units to be transmitted according to the system of the present invention can be smaller than in the standard GSM-system. This is advantageous in view of the interleaving depth. If, for example, two OFDM/TDMA-timeslots are mapped into one GSM-timeslot, and 8 OFDM/TDMA-timeslots form one frame (8-TDMA), an interleaving depth of 8 frames (same as GSM) results in a total interleaving delay of 18.461 ms, which is half of the corresponding total interleaving delay of 36.923 ms in the GSM-system. Therefore, the transmission of information in a system according to the present invention can have a smaller overall delay with the same interleaving (frequency and interference diversity). An interleaving depth of 16 frames (approximately 36.923 ms) results in the same overall delay as in the standard GSM-system, but is much more reliable in view of transmission problems (time-, frequency- and interference diversity). For the transmission of speech signals, usually a smaller interleaving delay is desired due to the real time requirements. For example, for the transmission of speech signals interleaving depths smaller than 40 ms and short time-frames (4-10 ms) are advantageous. For the transmission of data signals, the real time requirements are not so important, so that a longer interleaving depth can be chosen to improve the data transmission reliability.
Advantageously, the signals to be transmitted according to the present invention are interleaved with a total interleaving delay corresponding to 8xc3x978 GSM-timeslots. Alternatively, the signals to be transmitted according to the present invention may be interleaved with a total interleaving delay corresponding to 4xc3x978 GSM-timeslots.
In the transmission technique according to the present invention, allocating the subcarriers is brought about by generating a clock, producing the number of subcarriers according to the clock, transforming the subcarriers into time range bursts, and generating the OFDM/TDMA-timeslots by adding a guard time, a ramp time and an adaptation guard time to each of the time range bursts.